In a disc system, in order to play information recorded on an optical disc, the optical disc is irradiated with a laser beam, and reflected light is detected. In present optical disc devices, particularly DVD players and the like, it is necessary to play, by a single player, as many discs as possible among existing various kinds of discs such as single-layer DVD, double-layer DVD, DVD-RAM, DVD-R, DVD+R, DVD-RW, DVD+RW, CD-DA, CD-ROM, CD-R, and CD-RW. Therefore, it is necessary to recognize the type of disc at startup of the optical disc system or at disc change, and optimize the system setting according to the disc type for speedy playback.
There are established standards for DVDs and CDs, and it is desired that disc carriers are manufactured within the standards. Under present circumstances, however, discs out of the standards are also distributed due to variations in manufacturing process or the like. Especially, the detection sensitivity of an off-track signal is greatly affected by a difference in modulation degrees of playback signals due to a difference in track pitches or a difference in reflectivities, and therefore, there appear some discs that are inapplicable to the conventional system in which the off-track sensitivity is fixed.
FIG. 6 is a block diagram illustrating a conventional optical disc device of this type. In FIG. 6, reference numeral 1 denotes an optical disc as a data recording medium on which an information signal is recorded on a spiral track or a concentric track. Reference numeral 2 denotes a rotation driver for rotary-driving the optical disc 1; numeral 3 denotes an optical pickup for forming a light spot by focusing a light beam on an information surface of the optical disc, and detecting the reflected light beam, thereby to output various kinds of information; numeral 4 denotes a tracking actuator for moving the optical pickup 3 in the direction of the radius of the optical disc 1; numeral 5 denotes a tracking driver for driving the tracking actuator 4 on the basis of the output of a tracking controller 7; numeral 6 denotes a tracking error signal detector for generating a tracking error signal indicating a deviation of the optical pickup 3 from the track, on the basis of the output of the optical pickup 3; numeral 7 denotes a tracking controller for controlling the tracking driver 5 on the basis of the output of the tracking error signal detector 6 or an off-track signal detector 11; numeral 8 denotes a sled controller for controlling a sled driver 9 on the basis of the output of the tracking error signal detector 6; numeral 9 denotes a sled driver for driving a sled motor 10 on the basis of the output of the sled controller 8; numeral 10 denotes a sled motor for moving the optical pickup 3 on the basis of the output of the sled driver 9; and numeral 11 denotes an off-track signal detector for detecting an off-track signal indicating that the optical pickup 3 is off the track, on the basis of the output of the optical pickup 3.
Hereinafter, the operation of the conventional optical disc device constituted as mentioned above will be described.
When performing disc playback, the tracking error signal detector 6 detects a tracking error signal on the basis of the output of the optical pickup 3. Then, the tracking controller 7 performs processing for stabilizing tracking servo on the basis of the obtained tracking error signal, and the tracking driver 5 drives the tracking actuator 4 to execute tracking servo so that the actuator 4 follows the track.
Since the movable range of the optical pickup 3 in the disc radius direction is narrow, sled control is carried out to compensate for it. That is, the sled controller 8 obtains an output according to a deviation, i.e., a displacement in the disc radius direction, on the basis of the output of the tracking error signal detector 6, and the sled driver 9 drives the sled motor 10 on the basis of the deviation so as to cancel the deviation.
Further, in the conventional optical disc device, the off-track signal obtained by the off-track signal detector 11 is used for judgement on off-track due to vibration, or track pull-in, and detection of track cross direction at accessing. When this off-track signal is used for recovery of off-tracking, it is output to the tracking controller 7, and the tracking controller 7 outputs a control signal for compensating the off-tracking to the tracking driver 5, whereby the tracking actuator 4 drives the optical pickup 3 so as to put the optical pickup 3 back to the original track.
An off-track signal generation method by the off-track signal detector 1 will be described with reference to FIGS. 7(a) and 7(b). FIGS. 7(a) and 7(b) are schematic diagrams illustrating waveforms obtained when the optical pickup 3 crosses the track. FIG. 7(a) shows an optical disc playback signal obtained by the optical pickup 3, and an off-track signal generated by the playback signal. Generation of the off-track signal is as follows. That is, as shown in FIG. 7(a), a lower envelope signal e of the playback signal is compared with a reference signal r indicated by a broken line, and an off-track signal as shown in FIG. 7(a) is generated according to the comparison result.
Generally, utilizing “hollows” in the playback signal which are generated when the optical pickup 3 crosses the track, i.e., portions where the lower envelope signal e of the playback signal is convex upward, portions where the lower envelope signal exceeds a reference value of the reference signal r are regarded as “high” while portions lower than the reference value are regarded as “low”, thereby generating an off-track signal. As described above, the condition of the playback signal when the optical pickup 3 crosses the track depends on the disc carrier, and further, the degree of demodulation also varies depending on the track cross speed. FIG. 7(b) shows a waveform in which “hollows” in the playback signal at track crossing are relatively small, indicating that the output sensitivity of the off-track signal shown in FIG. 7(b) becomes lower than that shown in FIG. 7(a).
Particularly, the off-track sensitivity is significantly affected by crosstalk, i.e., interference with an adjacent track, which is caused by a narrow track pitch or a pit formation defect. Since the off-track signal is used for off-track detection and track pull-in judgement as described above, if the off-track signal cannot be normally detected, false judgement might be made on track following, whereby stability of the system is lost.
In order to solve these problems, various kinds of off-track signal detectors have already been developed.
FIG. 8 is a block diagram illustrating a conventional off-track signal detector disclosed in Japanese Published Patent Application No. Hei. 9-219027. FIG. 9 shows operation waveforms thereof.
In FIG. 8, a track error signal 104 is at zero level in the center of a track formed on an optical disc, and the error signal increases in the positive or negative direction with decreasing proximity to the center of the track.
A zero-cross detection circuit 105 detects a zero cross of the track error signal 104 to generate a zero-cross pulse 106. A sample/hold control circuit 107 controls the sample/hold timings of a peak level sample/hold circuit 110 and a bottom level sample/hold circuit 111 by using the zero-cross pulse 106.
As shown in FIG. 9, the track error signal 104 attains a zero level and an optical detector sum signal 101 attains a low level in a groove section in the center of a track on the optical disc, and the track error signal attains a zero level and the optical detector sum signal 101 attains a high level in a land section at the boundary between a track and an adjacent track. Accordingly, it is possible to detect a peak level and a bottom level of the optical detector sum signal 101 by detecting a zero cross of the track error signal 104 and sampling/holding the optical detector sum signal 101 at that timing.
When a zero cross of the track error signal 104 is detected, judgement as to which of peak and bottom is to be sampled/held is carried out using the off-track signal. That is, the sample/hold control circuit 107 detects a zero cross pulse 106 of the track error signal 104, and outputs a bottom sample/hold pulse 109 when the off-track signal 103 at this time is low (on-track state), and outputs a peak sample/hold pulse 108 when the off-track signal 103 is high (off-track state). Thereby, the peak level sample/hold circuit 110 detects an optical detector sum signal peak level 112 of the optical detector sum signal 101, while the bottom level sample/hold circuit 111 detects an optical detector sum signal bottom level 113 of the optical detector sum signal 101, and an intermediate level generation circuit 114 generates an intermediate level between the optical detector sum signal peak level 112 and the optical detector sum signal bottom level 113. A comparator 102 compares the optical detector sum signal 101 with the intermediate level as a reference value, and outputs an off-track signal 103 according to the comparison result.
However, even the off-track detection circuit disclosed in Japanese Published Patent Application No. Hei. 9-219027 cannot accurately detect off-track signals from all kinds of optical discs, and it is unavoidable that different sensitivities are provided for different discs. As a result, it is difficult to obtain a stable off-track sensitivity of the system.
As described above, in the conventional disc system, it is difficult to keep the off-track sensitivity constant for various kinds of discs, and accordingly, it is difficult to secure stabilities of off-track detection and track pull-in judgement.